The present invention relates to optical communication networks and, more particularly, to optical devices for routing multi-wavelength optical signals.
When multiple users share a transmission medium, some form of multiplexing is required to provide separable user sub-channels. There are many multiplexing techniques available that simultaneously transmit information signals within the available bandwidth, while still maintaining the quality and intelligibility that are required for a given application. Optical communication systems, for example, increasingly employ wavelength division multiplexing (WDM) techniques to transmit multiple information signals on the same fiber, and differentiate each user sub-channel by modulating it with a unique wavelength of invisible light. WDM techniques are being used to meet the increasing demands for increasing speed and bandwidth in optical transmission applications.
In optical communication networks, such as those employing WDM techniques, individual optical signals are often selectively routed to different destinations. Thus, a high capacity matrix or cross-connect switch is often employed to selectively route signals through interconnected nodes in a communication network. Many cross-connect switches used in optical communication networks are either manual or electronic, requiring multiple optical-to-electrical and electrical-to-optical conversions. The speed and bandwidth advantages associated with transmitting information in optical form, however, makes an all-optical network the preferred solution for WDM-based optical networks. Moreover, all-optical network elements are needed to provide the flexibility for managing bandwidth at the optical layer (e.g., on a wavelength by wavelength basis). In addition, it is often desirable to remove light of a given wavelength from a fiber or add light of a given wavelength to the fiber. A device that provides this feature is often referred to as a wavelength add-drop (WAD) multiplexer.
Wavelength blockers are optical devices that accept an incoming signal of multiple wavelength channels and independently pass or block each wavelength channel. Wavelength blockers can be used as components in a larger optical communication system, for example, to route a given optical signal along a desired path between a source and destination. Optical cross-connect switches and wavelength add-drop multiplexers, for example, could be implemented using wavelength blockers. A wavelength blocker provides a number of desirable features. First, a network element using wavelength blockers is modular and thus scalable and repairable. Second, network elements using wavelength blockers have a multicasting capability. Third, wavelength blockers are relatively easy to manufacture with high performance. For example, wavelength blockers have only two fiber connections, and it is possible to use a polarization diversity scheme to make them polarization independent.
As the demand for optical bandwidth increases in WDM communication systems, it is desirable to increase the number of channels. Unfortunately, an increase in the number of channels provides a corresponding increase in the size, cost and insertion loss of the optical devices in such WDM communication systems. A need therefore exists for improved wavelength blockers that permit optical cross-connect switches, wavelength add-drop multiplexers and other optical devices to be fabricated with reduced size and cost. A further need exists for two-port wavelength blockers that permit optical cross-connect switches and wavelength add-drop multiplexers to be configured without complex waveguide crossings. Yet another need exists for improved wavelength blockers having a frequency spectrum with a generally flat transmission spectrum in both amplitude and phase.
Generally, a method and apparatus are disclosed for filtering an input wavelength-division multiplexed (WDM) signal comprised of N wavelength channels. The disclosed wavelength blocker includes a demultiplexer for producing a plurality of demultiplexed output signals from the input WDM signal and a multiplexer for producing an output WDM signal. In addition, a shutter array selectively passes each of the N wavelength channels using a plurality of shutters. According to one aspect of the invention, the demultiplexer is coupled to the multiplexer using a plurality of waveguides having approximately equal length, in order to reduce multipath interference.
The shutters may be embodied, for example, as Mach-Zehnder switches, electro-absorption modulators or Y-branch switches. Each of the N wavelength channels in the incoming signal are selectively passed or blocked using a thermo-optic or electro-optic control signal to control the state of the corresponding shutter. According to another aspect of the invention, crosstalk among the various N channels can be reduced using dilation techniques that position two shutters in series, especially where the shutters are thermo-optic Mach-Zehnder switches. The disclosed wavelength blockers may be utilized in wavelength-selective cross connects and wavelength add-drop multiplexers, as well as other optical devices.
A more complete understanding of the present invention, as well as further features and advantages of the present invention, will be obtained by reference to the following detailed description and drawings.